Oslo Company prepared the following contribution format income statement based on a sales volume of $1,000$ units $($the relevant range of production is $500$ units to $1,500$ units$)$:

Sales $ $20,000$

Variable expenses $12,000$

Contribution margin $8,000$

Fixed expenses $6,000$

Net operating income $ $2,000$

$15.$ Assume that the amounts of the company$$s total variable expenses and total fixed expenses were reversed. In other words, assume that the total variable expenses are $$6,000$ and the total fixed expenses are $$12,000.$ Using the degree of operating leverage, what is the estimated percent increase in net operating income of a $5\%$ increase in unit sales?